In many audio and video devices, an audio circuit includes an electret microphone and a low noise preamplifier into which a signal is input from a microphone. The preamplifier output signal may then be converted into a digital signal in an analog-digital converter and then amplified, processed, filtered and possibly memorized or stored in this form.
Electret microphones must be polarized by a positive DC voltage through a pull-up resistor initializing the microphone operating state. The microphone output signal, formed by a variable current, passes through this pull-up resistor so that the variable voltage at the terminals of this resistor is representative of the microphone output signal. One of the main problems with this type of audio circuit is the lack of immunity of the useful microphone output signal to noise affecting the positive DC biasing voltage of the electret microphone.
One known method of correcting this problem is to filter the biasing voltage before applying it to the pull-up resistor. This filtering may be done for example using an R.C. circuit. This process is expensive because it requires the addition of an additional capacitor external to the integrated circuit, and also it is not very efficient, particularly at the low frequencies present in the audio range.